A mixer is a three port network which translates an input signal at one frequency to another frequency. To affect this translation, the input signal is heterodyned or "mixed" with another input signal by a non-linear device. This process generates two primary output signals having frequencies which are equal to the sum of and the difference between the frequencies of the two input signals. However, unwanted mixing products are usually also generated. In a turner of a television or radio receiver, a radio frequency (RF) signal received form a RF stage is mixed with a local oscillator (LO) signal generated by a local oscillator to produce an intermediate (IF) signal.
An excellent description of various types of mixers, their operation and their relative advantages and disadvantages is provided in Introduction to Radio Frequency Design, by W. H. Hayward, published by Prentice Hall, copyright 1982, pages 232-245.
Some mixers utilize an active device, such as a transistor configured as an amplifier, while others utilize a passive device, such as diode. Passive mixers have an advantage over active mixers in that active mixers tend to amplify unwanted mixing products, making it more difficult to remove them. The mixing devices of mixers may be operated in a "square" mode or in a "switching" mode. Switching mode mixers tend to generate fewer unwanted products than square law mixers. Of the switching mode mixers, so called "balanced" mixers provide unique advantages, among them being the elimination of at least one of the two input signals at the other input and at the output of the mixer. This and other advantages of balanced mixers will be discussed in greater detail below.
The above identified patent application of Muterspaugh and Anderson discloses a singly-balanced mixer. RCA brand color television receivers including CTC-156 and 157 chassis, documented in "RCA/GE Color Television Service Data--CTC 156/157", published by Thomson Consumer Electronics, Indianapolis, Ind. in 1989, employ tuners of the TCCR and TCHR type with a singly-balanced mixer similar to the one disclosed in the Muterspaugh and Anderson patent application. These singly-balanced mixers include a single pair of mixing diodes and a single balun for coupling the LO and RF signals to the mixing diodes in a balanced impedance configuration.
U.S. Pat. No. 4,601,063 of Price discloses a doubly-balanced mixer. RCA brand color television receivers including CTC-133 chassis, documented in "RCA Color Television Basic Service Data CTC 133 Series", published by RCA Corporation, Indianapolis, Ind. in 1985, employs a doubly-balanced mixer similar to the one disclosed in the Price patent. These doubly-balanced mixers include two pair of mixing diodes arranged in a bridge configuration and two baluns for coupling respective ones of the LO and RF signals to the mixing diodes in a balanced impedance configuration. Each pair of diodes are biased by a current source network including a voltage supply and a resistor coupled between the voltage supply and the junction of the two diodes.